I am trying to upgrade through the "Update Manager." After clicking upgrade the "Distribution Upgrade" dialog appears and begins, "Preparing to Upgrade" at which point I get a message:

Third party sources disabled
Some third party entries in your sources.list were disabled. You can re-enable them after the upgrade with the 'software-properties' tool or your package manager.

I click close and the "Distribution Upgrade" moves on to "Setting New Software Channels." At that point I get:

Error during update
A problem occurred during the update. This is usually some sort of
network problem, please check your network connection and retry.
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/karmic-backports/main/source/Sources 404 Not Found [IP: 91.189.91.13 80]
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/karmic-backports/restricted/source/Sources 404 Not Found [IP: 91.189.91.13 80]
W:Failed to fetch h t t p://us.archive.ubuntu.com/ubuntu/dists/karmic-backports/universe/source/Sources 404 Not Found [IP: 91.189.91.13 80]
W:Failed to fetch h t t p://us.archive.ubuntu.com/ubuntu/dists/karmic-backports/multiverse/source/Sources 404 Not Found [IP: 91.189.91.13 80]
E:Some index files failed to download. They have been ignored, or old ones used instead.